shephard/lab mix puppy

Question
Hello,
    Last week we brought a 5ish month old male unneutered Lab/shepherd mix puppy into our family. Murphy came to us through someone that knew someone that knew someone that was just going to put him out in the street. When we brought Murphy home he was underweight, extremely gassy, a bit nervous, and he was extremely possessive about his food(growl and bark if anyone comes near his food while he is eating), aggressive eater (Eating his food like its his last meal, which we believe came from previous owners feeding him sporadically or if at all). In the week he has been with us we have taken care of the underweight and gassy issue by switching  to Iams for puppies. Murphy has been taking walks on a leash and enjoys showing off his new "sit" trick. Murphy now doesn't attack his food on sight, we have taught him to sit until given permission to eat.  He is going to the vet in a few days to start his shots (as far as we know he has never been to a vet). Once shots are taken care of we will be enrolling him in obedience training. Now my question is: What can we do about the growling and aggressive eating in the meantime? We would like to start working on this before he gets any older.

We use firm voice with "NO", but that doesn't seem to be working.  Trying to pet him while he is eating seemed to just upset him and make him eat even faster.
Thank you for any ideas or help you can give us.

Answer
I am old school enough that this method bothers me a little, but it is in my Puppy Raising manual along with a bunch of well proven stuff.  

Don't put all of the dogs food in the dish to start.  When it finished what you gave it, add more.  After a couple of additions, the dog will come to welcome you getting near its food dish.

Note, one of the well proven ideas in the manual is switching to adult chow at 4 months.  Make your next bag of chow, the adult version of the Iams puppy chow.  the adult chow slows growth allowing more time for the joints to develop under less stress.  Keep him lean too, http://www.longliveyourdog.com/twoplus/RateYourDog.aspx

If he has survived a lack of shots, one combo shot will put him in good shape.  Many early shots are useless because immunity from the mother keeps them from taking.